friend class world;

typedef unsigned int id_t;

typedef unsigned int evolution_t;

person(void): id_(0), evolution_(0) {}

friend std::ostream& operator<<(std::ostream& o, person const& p) {

return o << "person(" << p.id_ << ", " << p.evolution_ << ")";

}

id_t id_;

evolution_t evolution_;

typedef unsigned int id_t;

world(void): next_id_(1) {}

void add_person(person const& a_person);

friend std::ostream& operator<<(std::ostream& o, world const& w) {

o << "world(" << w.next_id_ << ", {";

BOOST_FOREACH(person const& p, w.persons_) {

o << " " << p << ", ";

}

return o << "})";

}

id_t next_id_;

std::vector<person> persons_;

persons_.push_back(a_person);

// This block must be no-throw.

//[scope_exit

person& p = persons_.back();

person::evolution_t checkpoint = p.evolution_;

SCOPE_EXIT(const bind checkpoint, const bind& p, bind this_) {

if (checkpoint == p.evolution_) this_->persons_.pop_back();

} SCOPE_EXIT_END

//]

// ...

checkpoint = ++p.evolution_;

// Assign new id to the person.

world::id_t const prev_id = p.id_;

p.id_ = next_id_++;

SCOPE_EXIT(const bind checkpoint, const bind prev_id, bind& p,

bind& next_id_) {

if (checkpoint == p.evolution_) {

next_id_ = p.id_;

p.id_ = prev_id;

}

} SCOPE_EXIT_END

// ...

checkpoint = ++p.evolution_;

person adam, eva;

std::ostringstream oss;

oss << adam;

std::cout << oss.str() << std::endl;

BOOST_TEST(oss.str() == "person(0, 0)");

oss.str("");

oss << eva;

std::cout << oss.str() << std::endl;

BOOST_TEST(oss.str() == "person(0, 0)");

world w;

w.add_person(adam);

w.add_person(eva);

oss.str("");

oss << w;

std::cout << oss.str() << std::endl;

BOOST_TEST(oss.str() == "world(3, { person(1, 2), person(2, 2), })");

return boost::report_errors();